Many commercial chemical processes rely upon suspending solid particles in a fluid, or combining two or more immiscible fluids in such manner that one forms as droplets or gas bubbles suspended in the other.
The particles, droplets, or gas bubbles (hereinafter sometimes referred to as "objects" for convenience) then undergo some reaction or physical change, such as polymerization, the success of which is dependent upon the sizes of the objects in suspension. If the sizes of the objects can be measured in-situ, adjustments can be made in a process during the progress thereof to optimize the resulting product. However, in-situ measurement of the size of objects may not always be possible or convenient by conventional means. For example, in processes wherein a medium includes liquid droplets or bubbles whose size is critical to the product produced from such process, measurement of the size often requires the removal of a sample of the medium from the reaction or mixing vessel. However, it is difficult, or impossible, to remove and analyze such a sample without causing a change in the droplet or bubble size.
In the absence of suitable methods for in-situ measurements of the sizes of objects, it is not uncommon for adjustments in a process to be made on an empirical basis. The necessity of relying upon empirical techniques is inherently wasteful of both time and materials, and unduly increases production costs.
An object of the present invention, therefore, is to provide a method for the direct, in-situ measurement of the sizes of objects present in a fluid medium and thus improve production efficiencies by eliminating some empirical adjustments.
A co-pending application, U.S. Ser. No. 756,359, now abandoned, of the same assignee discloses a method for in-situ determination of the sizes of objects, present in a fluid medium and is based on measuring the intensity and variance in intensity of light reflected from such objects. In that method the intensity of the reflected light depends on the refractive index of the medium containing the objects. Therefore, successful application of such method requires that the refractive index of the medium remain substantially constant during the measurements.
The present method comprises the measurement of the intensity and variance of light scattered by particles, droplets, or bubbles present in a transparent or translucent fluid medium. References hereinafter to scattered light include Raman scattered light and light absorbed by and reemitted from objects or liquids at a frequency different from that of the illuminating light.
One of the differences between the method of this invention and that disclosed in the above mentioned co-pending application is that scattered light is not affected to any substantial degree by changes in the refractive index of the medium. Thus, the present method is applicable in those instances in which the refractive index of the medium is variable.
Another advantage of the present method is that it enables the measurements to be restricted to the light scattered by specific substances only, whereas the reflected light method disclosed in the aforementioned application measures light reflected from all particulate matter in the sample. Thus, by using the method of the present invention it is possible to measure the size of objects of a specific chemical composition in a medium containing other materials of different compositions, provided the other materials do not severely affect the transmission of the illuminating light and the scattered light.